Smart cards, which look and feel much like ordinary credit cards, contain circuitry for communicating, processing and storing information. Smart cards have many applications, but primarily, smart cards are used for financial transactions. The smart card stores a monetary balance which is credited/debited with each transaction. Additional uses for smart cards are as identification tags for employees, etc.
To provide communication between the smart card and a card reader, smart cards have been adapted with metalized tabs on the card surface which provide a direct connection between the circuitry within the card and the card reader. With this arrangement to make a transaction, the card is inserted into a receptor slot of the card reader where reader probes contact the metalized tabs. The reader provides a controlled and filtered power supply and data signal to the smart card via the metalized tabs.
The advantage of providing the smart card with metalized tabs is that very clean, filtered and separated power and data signals may be communicated to the smart card. This ensures proper operation. However, there is a disadvantage in that the card must make contact with the card reader, i.e., the metalized tabs must contact the card reader probes. In an improved arrangement, the smart card is adapted to operate in a "contact-less" manner.
Contact-less smart cards have been proposed and implemented with success. The contact-less smart card is remotely powered and communicated with by the card reader. Typically, power to the smart card is supplied by a high frequency signal and a data carrier signal is supplied by another frequency, which is preferably divided directly from the power signal. With this arrangement, the user simply positions the smart card near the reader to perform a transaction. The card need only come within 10 or 15 centimeters (cm) of the card reader. Power and data signals are inductively coupled from the reader to the card using two tuned resonant circuits. The power coupling frequency is preferably unmodulated and spectrally pure so as not to electrically jam or interfere with any electronic equipment that operates on adjacent frequency bands. The data carrier frequency is a submultiple of the power coupling frequency and is modulated using a suitable modulation technique such as amplitude shift keying (ASK). The data carrier is coupled into the card using an inductor or coil in each of the card and the card reader. Again to avoid interference, the data carrier signal level is held to a very low level.
One problem with contact-less smart cards is recovering the data signal from the data carrier. Because of the relatively large value of the power signal as compared to the data signal there is a significant power signal component present on the data coil. This component of the power signal must be removed in order to accurately recover the data signal. Also, the rectification and regulation of the power signal to generate the power supply results in a substantial power signal frequency component on the power supplies, requiring a data carrier recovery circuit with good power supply rejection.
A prior proposed solution to this problem requires use of a high order filter which because of the proximity of the power and data signals in frequency requires many poles of filtering and a high filter gain for recovery of the data signal. In this approach, the data carrier signal is first filtered to remove the interfering power signal. Then, the data carrier signal is limited to facilitate detection of the data. In addition, this approach requires expensive, precise analog components to implement the filtering, and many stages of both filtering and gain to achieve the necessary signal rejection and gain. Furthermore, the number of components consume a considerable amount of power reducing the distance from the reader that the device may be effectively used. As a result, the proposed solution is not desirable for use in smart card devices.
Another proposed solution uses mixers to mix the input signal with a clock at the data carrier frequency. This mixed signal is then filtered and amplified to retrieve the data. This method provides improvement over the use of multi-pole filters and multiple stage gain by reducing the amount of filtering required. Still there remains a need for an efficient solution to the problem of accurately recovering the data signal in view of inherent power signal noise which is easily implemented in smart card technology.
Data detection is further hampered as a result of the variability of the characteristics of the received communication signal and particularly the strength of the received signal. As an example, in a system employing amplitude shift keying (ASK) with 100%/0% modulation (i.e., on/off keying), the bandwidth of the system limits the communication range of the card to the reader. Increasing bandwidth results in a reduction of the quality factor or "Q" of the data detection circuit, and hence limits the communication distance. Decreasing the data rate is not desirable in that it increases transmission time. The bandwidth limitation causes a spreading of the envelope of the ASK signal by filtering the transitions from "0" to "1" and from "1" to "0". Data detection in an ASK system requires a data detection threshold to determine if the signal is a "1" or a "0" taking into account this bandwidth limitation. Because of variation in the strength of the detected signal if the data detection threshold is set, for example, too low for a strong signal or too high for a small signal decoding errors will occur. Typically a fixed data detection threshold is set to provide best data detection at a set distance of the card to the reader. However, it is desirable to maximize the range over which the card and the reader may accurately exchange data.
One prior art attempt to maximize range provides for taking an average of the data levels to determine a threshold level for data detection. However, averaging requires encoding data such that there is always a transition, such as Manchester coding, which reduces the data rate by at least one half. Therefore, there remains a need for a wireless powered communication device and reader system which provides accurate data detection over a wide range of distances of card to reader.